scons: tweak logic for whether a path is a 'system' path, avoiding case where a custom path like /opt/mapnik/lib would be assumed to be a standard system path
scons: tweak logic for whether a path is a 'system' path, avoiding case where a custom path like /opt/mapnik/lib would be assumed to be a standard system path

git-svn-id: 5adfda8227e14e59fd47d5bd9925a30df16fed74@2089 d397654b-2ff0-0310-9e29-ba003691a0f9

craigds/mapnik2,craigds/mapnik2,makinacorpus/mapnik2,makinacorpus/mapnik2,makinacorpus/mapnik2,craigds/mapnik2,makinacorpus/mapnik2,craigds/mapnik2